Add wxNO_UNSAFE_WXSTRING_CONV2 macro

The macro disallows implicit conversions between wxString and const
char*

include/wx/gtk/dataform.h

@@ -24,7 +24,9 @@ public:
     // we have to provide all the overloads to allow using strings instead of
     // data formats (as a lot of existing code does)
     wxDataFormat( const wxString& id ) { InitFromString(id); }
+#ifndef wxNO_IMPLICIT_WXSTRING_ENCODING
     wxDataFormat( const char *id ) { InitFromString(id); }
+#endif
     wxDataFormat( const wchar_t *id ) { InitFromString(id); }
     wxDataFormat( const wxCStrData& id ) { InitFromString(id); }